1. Field of the Invention
The present invention relates to a line protection switching control system in a duplexed fiber interface shelf. More particularly, it relates to a system, which is provided on an ISDN network and in which APS status can easily coincide between duplexed devices for switching transmission lines by the means of APS (Automatic Protection Switching).
2. Description of the Related Art
Commercial use of broadband-ISDN systems has started, and ATM switches employed for the systems have actively been developed in recent years. Referring now to FIG. 26, a structural example of a broadband-ISDN system is shown.
In FIG. 26, switches 1 mean ATM switches as an example. The ATM switches 1 are connected by transmission highways, for example, optical fiber lines 130. The switches 1 are also connected via a local switch, that is a concentrator, 131, or directly connected to the subscriber lines (user) or toll switches 132.
FIG. 27 is a structural block diagram, which shows an example of the ATM switch 1 illustrated in FIG. 26. The ATM switch 1 illustrated in FIG. 27 is formed of duplexed ATM switch modules 2 for exchanging ATM cells, a duplexed fiber interface shelf (FIFSH) 3 having duplexed fiber interface common cards 4 for controlling network highways connected to other switches and duplexed fiber interface cards (interface section) 5, duplexed subscriber line controllers 6 for controlling subscriber lines, duplexed network control signal processors 7, and an operation system 8.
In FIG. 27, the duplexed fiber interface cards 5 of the fiber interface shelf (FIFSH) 3 are constructed individually, according to an interface with each of the network highways, i.e. and is a fiber interface card for terminating a line format, so called as an individual section. The duplexed fiber interface common cards (FIFCOM) 4 control highways in common, which are known generically as common section. Further, the duplexed fiber interface common cards 4 have interfaces with the ATM switch modules 2.
In the above-described structure of ATM switch 1, network highways 130 linking between the ATM switches are duplexed. Simultaneously, the ATM switch module 2, the fiber interface common card 4, subscriber line controller 6, and a network control signal processor 7 respectively have a duplexed structure, to continue the communication by switching the status of ACT (active) or STB (standby), according to APS (Automatic Protection Switching) as a line protection system of network highways.
In this example, the APS means a system for self-controlling the line switching, without the control of operation system (OS) 8, in concurrent with detecting line faults. The line processing device self-controls switching of lines, so that the interval from the time of detecting faults to the time of completion of the switching can become shorter (less than 50 msec). The structure of the APS is prescribed in the Bellcore Recommendation TR-NWT-00253 or the like. It is also defined for the APS system to switch transmission lines with commands sent from the operation system (OS) 8. Levels of several ranks are provided for reasons of faults, a switching command or the like. These levels have predetermined priorities.
SONET (Synchronous Optical NETwork) is well known in North America, as an optical network for synchronizing an optical signal having the speed of 45M bits in the third-order group on digital hierarchies. K1 or K2 bytes in an overhead of a SONET signal frame is used to send and receive protocols to and from other switches to be connected.
In the structure of the fiber interface shelf 3, which is above discussed, the APS is performed by controlling, judging the priority, and administrating the status in the fiber interface common card 4, and switching the duplicated fiber interface cards 5.
In this example, the system can continually keep the operation of the duplicated fiber interface common cards 4 of both the ACT and STB groups in common, by performing same operation without distinguishing the ACT or STB group in the fiber interface common cards of the both duplexed groups.
However, there is a fault, which can be detected only by the common section of one group, among line faults including faults of the line devices itself detected by the fiber interface common card 4, i.e., common section. This will be explained in accompany with FIG. 28.
In FIG. 28, communication lines a to d are connected to the fiber interface common cards 4, i.e., common section and the fiber interface cards 5, i.e., individual section. The communication lines a and b are employed as a WORKING and a PROTECTION lines in a common section of #0 group, respectively. The lines c and d are employed as a WORKING and a PROTECTION lines in a common section of #1 group, respectively.
This means that when a fault F is generated in the communication line a, as illustrated in FIG. 28, the common section of #0 group recognizes it as a fault in the ACT (active) group, and performs line switching from the working line to the protection line. However, the common section of #1 group cannot detect this fault, which occurs in the #0 group.
If only one common section of one group can detect a fault as described above, the problems will be brought as follows. At first, the APS condition of the group, which has detected the fault becomes different from that of the other group, which has not detected the fault. Therefore, even if the fault is detected in only an ACT group, the switching can be normally performed. However, the common section in the SBY group cannot detect the fault, so that the APS condition of the SBY group becomes different from that of the ACT group.
If only the SBY group detects a fault, both the OS and the individual section act on the basis of the condition of common section in the ACT group, so that the APS condition is varied without switching actually.
Further, when initial driving or booting is performed from a power OFF state or the like, the fiber interface card 5 cannot recognize what the current condition in controlling the APS is. It has a counterbalancing disadvantage of the inconsistency between condition of the ACT and SBY groups, unless the condition of the common section for the ACT group is not in initialization or booting.
Furthermore, in FIG. 28 as described above, the protection group is employed due to the fault on a working group in the common section of #0 group, while the working group is employed without switching requirements in the common section of #1 group. Under the condition, when switching of common sections from the #0 group to the #1 group is performed, line
switching from the protection line to the working line is performed due to the APS condition in the common section of the #1 group.
Accordingly, it is required to reconcile conditions between the ACT and SBY groups by copying information from the fiber interface common card 4 of the ACT (active) group to the fiber interface common card 4 of the SBY (standby) group. However, it is complex to copy the information of the condition from the ACT group to the SBY group, and it should be also considered that the condition may be varied while copying.
For example, as shown in FIG. 29, if the condition of the APS of the #1 group is copied to the fiber interface common card 4, i.e., a common section of the ACT group and line switching is performed, the common section in the ACT group cannot detect the restoration of a fault, even if the fault, which occurred in the connecting line c, has been restored. Thus, the APS condition in the common section of the ACT group cannot be changed.
When a fault is detected in only the SBY group, and the APS condition is copied to the ACT group, it is required to copy the APS condition from the SBY group to the ACT group when the common section of SBY group detects the fault restoration.
Meanwhile, the APS architecture has 1+1 and 1:n protection switching structures. Each of the structures has a uni-directional transmission mode and a bi-directional transmission mode. In the 1:n protection switching structure, one protection channel (line) is provided for n working channel (line).
In the 1+1 protection switching structure, a signal flows to both the working and protection lines. A selector selects either the working or protection line. In this example, as shown in FIG. 30, a bridge 22 and selector 23 are provided between devices, which support 1+1 structured bi-directional transmission mode. The bridge 22 is connected to both working and protection lines, and the selector 23 faced to the bridge 22 is switched and connected to the working or the protection line.
In this 1+1 line structure, the bridge 22 is fixedly connected to both the working and protection lines. Therefore, the switching can be completed immediately at the bridge 22. While the selector 23 sends a request for switching to a faced office using the K1 byte, a line number of K2 byte, which indicates a bridge completion is returned as a response, and the selector 23 confirms the correspondence between the line number of the sent K1 byte and that of the received K2 byte and completes switching at last.
This sequence is shown in FIG. 31. In this example, the bridge 22 is used as a station A and the selector 23 is used as a station B. If a fault is detected in the station A, the station A sends a request for switching line to the station B. When the station B receives this request, the station B returns the acknowledgement of bridge completion to the station A. In this time, the A station switches the selector. Further, the station B sends a request for switching lines to the A station. When the station B receives the acknowledge of bridge completion sent from the station A, the selector switching is performed.
In this case, 1:n protection switching structure has the same protocol of the procedure of switching as that of 1+1 line switching structure. It is one of the drawbacks of the conventional system that it takes too much time to switch lines, because line Switching should be performed after receiving a response from the faced station.